1. Field of the Invention
The present invention relates to semiconductor devices, and more particularly to a semiconductor device in which secular deterioration of the electrical characteristics of semiconductor elements is prevented by a coating of silicone varnish.
2. Description of the Prior Art
In semiconductor devices, a variety of shielding means are generally provided in order to shield the surface of an element, especially the end of a P-N junction and the portion of an aluminum electrode, from external moisture etc. and to thus prevent the characteristics of the element from being deteriorated.
As the shielding means, there have hitherto been known resin sealing means and hermetic sealing means such as can sealing and ceramic sealing. The latter method, such as can sealing and ceramic sealing, is considerably expensive. In order to obtain products of low cost, the resin sealing means which is comparatively inexpensive is adopted.
In a practical example of a method of using the resin sealing means, an element after completion of the assembling steps of pellet mounting, wire bonding etc. is wrapped in an epoxy resin such as P 410B (Trade name of Morton Chemical Company), and the resin is solidified by thermal hardening at for example 150.degree. to 180.degree.C. According to Japanese Utility Model Application Publication No. 22590/1966 entitled "Semiconductor Device" , the surface of the element is undercoated with a water-repellent silicone resin before the sealing with the epoxy resin, whereby the shielding effect is more enhanced.
In the sealing with only the expoxy resin and even in the resin sealing accompanied by the undercoat treatment, however, the problem remains that when the semiconductor element operates especially in a high temperature state, e.g. 100.degree.C. to 150.degree.C. the deterioration of various characteristics of the element, such as a secular increase of the reverse current of the P-N junction, arises.
The inventors have surveyed the problem of the characteristic deterioration of the element from all angles, and has found the facts to be described hereunder. The water-repellent silicone resin, such as silicone varnish and silicone rubber, employed in the undercoat treatment contains the methyl radical (CH.sub.3 --) and the phenyl radical (C.sub.6 H.sub.5 --) as side chain radicals. In particular, the phenyl radial is a polar radical of high electronegativity. Due to the phenyl radical, a channel is formed in the surface of the semiconductor element in the state of a high temperature of about 100.degree.C. The deterioration of the element characteristics, such as increase of the P-N junction reverse current, is caused by the presence of this channel.